(6) Satoshi Kawata
Osaka University and RIKEN, Japan.

Title: Nano-plasmonics for Raman imaging

Abstract:

Optical imaging provides richer information than any other imaging technique based on topographic information. However, the spatial resolution in optical microscopy is restricted by the diffraction limits of the probing light, which is a few hundred nanometers for visible light. This problem can be overcome by combining the plasmonic effects of near-field techniques with optical Raman microscopy. In our previous work [1-4], we have shown that Raman scattering combined with near-field microscopy, i.e., tip-enhanced Raman spectroscopy (TERS), provides super spatial resolution far beyond the diffraction limits of the probing light, along with an enhanced scattering efficiency. This is due to the nano-sized evanescent field created in close proximity of the apex of a nano-metallic tip, which enhances the scattering from the sample molecules directly under the tip apex. The image quality, such as the contrast and the resolution, can be further improved by invoking higher-order scattering effects in Raman scattering process because it provides better confinement of light field than the linear scattering due to the nonlinear effects, and better contrast due to the suppression of the luminescence background. We have shown this by utilizing the near-field effects in coherent anti-StokeRaman scattering (CARS), and have obtained high quality image with a spatial resolution of 15 nm [5].

Apart from the TERS technique, which is based on optical microscopy, we have also predicted high quality nano-imaging through pure plasmonic techniques. Earlier we had shown that an array of metallic nanorods can provide subwavelength imaging, if the localized plasmons in the nanorods are resonantly excited [6]. While this structure is capable of subwavelength imaging, it has two major restrictions. One, it can only work with one particular wavelength; and two, the image is transferred for a short distance within the limits of near-field, and hence undetectable in the far-field. In order to overcome these limitations, we have recently proposed a nanolens that is made of stacked array of silver nanorods, which are arranged at tapered angles [7]. The stacked arrangement of silver nanorod arrays provides the plasmonic transfer of light energy at extremely low losses, making it possible to have a long distance image transfer. At the same time, this arrangement broadens the resonance bands to such an extent that a large region of visible frequency resonates with the local plasmons, providing the possibility of color imaging. Most importantly, the tapered arrangement of nanorod arrays in our design provides sufficient magnification of the image, so that the image of a nanostructure can be detected in the far field though usual optics and detectors, such as microscopes and CCD cameras. This technique has the potential to be an indispensible imaging tool for nano imaging tiny color objects in the far field.

[1] N. Hayazawa, et al., Opt. Commun. 183, 333 (2000).
[2] N. Hayazawa, et al., Chem. Phys. Lett. 376, 174 (2003).
[3] H. Watanabe, et al., Phys. Rev. B 69, 155418 (2004).
[4] P. Verma, et al., Phys. Rev. B 73, 045416 (2006).
[5] T. Ichimura, et al., Phys. Rev. Lett. 92, 220801 (2004).
[6] A. Ono, J. Kato and S. Kawata, Phys. Rev. Lett. 95, 267407 (2005).
[7] S. Kawata, A. Ono and P. Verma, Nature Photon. 2, 438 (2008).